Understanding the mechanisms of heterogeneous radical chemistry on organic surfaces
Organic films are ubiquitous in indoor and outdoor environments, forming on surfaces such as windows, walls, furnishings, and atmospheric particles. Chemistry occurring on such surfaces can significantly influence the fate and lifetimes of atmospheric pollutants, aerosol formation and contribute to human exposure to harmful degradation products.
Humans spend approximately 90 % of their time indoors. However, understanding the chemistry that occurs on indoor surfaces and its impact on air quality is understudied owing to the complexity of indoor surfaces. High surface-to-volume ratios increase gas-surface collisions, but chemical kinetics and molecular mechanisms for surface reactions are poorly understood, despite their importance becoming increasingly clear (Ault et al., 2020).
Filling this important gap in our knowledge of surface oxidation chemistry is important for many environmental problems including aerosol compositional effects impacting air quality (e.g. ozone and radical losses and pollutant transformation), climate (impacts on radiation and cloud formation), toxicity of inhaled particulate matter, polymer degradation, and our fundamental understanding of radical oxidation chain reactions.
This proposal aims to unravel the chemistry and reactivity of surface bound organic peroxyl radicals (RO2•), which form during photo- or thermally initiated oxidation of organic surfaces, targeting those relevant to indoor and outdoor atmospheric chemistry, i.e., organic coatings such as polymers or paints, organic films on atmospheric aerosols and soot, and human skin oils.
Previous work has suggested a conventional oxidation mechanism for surface degradation which proceeds via free radical intermediates, which rapidly form RO2• radicals in air. The RO2• radicals are usually assumed to abstract an H atom from a nearby molecule, generating a hydroperoxide ROOH and a carbon-centred radical (Smith et al., 2018). However, surface RO2• radicals are relatively unreactive. It has therefore been proposed that RO2• radicals undergo recombination, leading to either radical propagation and autoxidation or termination.
Our hypothesis is that RO2• recombination is unlikely under realistic conditions of organic surface oxidation in air. Their concentrations are low and diffusion in the organic surface is negligible. Under normal conditions surface RO2• should predominantly react bimolecularly with NO (Jenkin et al., 2019). Preliminary work in our laboratory on the oxidation of self-assembled organic monolayers (SAMs) on silica nanoparticles show that SAM degradation in ambient lab air is far greater than in synthetic zero air, or zero air with added NO. These results therefore suggest that surface RO2• radicals undergo some unidentified reactions at a faster rate than with NO at ambient concentrations.
We will tackle this problem by analysing products of surface RO2• reactions on different thin organic layers deposited on inert micro- /nano-particles. Experiments will be carried out under conditions that mimic realistic environmental exposures (i.e. T, RH, light). We propose to make these films from solid organic salts, with low temperature thermal initiators embedded in the organic films in order to initiate RO2• oxidation.
Reactions will be carried out in flow tubes packed with the organic film-coated particles. Solid state RO2• can be detected directly by EPR spectroscopy. Monitoring the evolution of RO2• in synthetic air with controlled composition will provide information about RO2• reactivity. We will explore reactions in atmospheres containing NO, •OH, HO2•, O3, and other common atmospheric constituents.
Trapping of radical intermediates formed in the reactions will be explored using radical traps with MS detection (Williams et al., 2022). These traps can be co-adsorb with the organic films prior to initiation.
At the end of the experiments, the products (and trapped radical intermediates) will be washed off the particle surface and analysed by conventional techniques, e.g., (HPLC)-MS. We will also monitor gas phase composition by PTR-MS and GC analysis. The combination of kinetic data from exposure to different reactive species, detection of trapped radical intermediates, and analysis of reaction products will provide fundamental insights into radical-driven surface processes that influence indoor/outdoor air quality, human exposure, and climate-relevant aerosol properties.
(taken from Ault et al., 2020)
During this research project the student will gain knowledge and experience of:
(1) Laboratory physical organic chemistry, including building a new flow reactor in order to investigate the surface chemistry of organic films under a range of atmospheric conditions.
(2) Use of a range of on and off-line gas-chromatographic and mass-spectrometric methods to detect, identify and quantify gas and aerosol phase products, including radical intermediates.
(3) Use chemical process box models to design and optimise experiments and to evaluate and interpret laboratory observations.
References
Ault et al., Indoor Surface Chemistry: Developing a Molecular Picture of Reactions on Indoor Interfaces. Chem, 6, 3203–3218, 2020; https://doi.org/10.1016/j.chempr.2020.08.023.
Smith et al., The Fate of the Peroxyl Radical in Autoxidation: How Does Polymer Degradation Really Occur? Acc. Chem. Res., 51, 2006−2013, 2018; https://doi.org/10.1021/acs.accounts.8b00250.
Jenkin et al., Estimation of rate coefficients and branching ratios for reactions of organic peroxy radicals for use in automated mechanism construction. Atmos. Chem. Phys., 19, 7691–7717, 2019; https://doi.org/10.5194/acp-19-7691-2019.
Williams et al., New Approach to the Detection of Short-Lived Radical Intermediates. J. Am. Chem. Soc., 144, 15969−15976, 2022; https://doi.org/10.1021/jacs.2c03618.